The principle of conservation of momentum can be used to calculate the velocity of objects after an explosion. This is indeed a confusing way of putting things. Law of Conservation of Momentum The law of conservation of momentum states that for two objects colliding in an isolated system, the total momentum before and after the collision is equal. This is the key step as it enables us to calculate the velocity of the two wagons after the collision. But the latter part should read: Momentum is always conserved but energy is not always conserved($^+$). Use cons of momentum for collisions (which happen very fast so you can ignore any outside forces). Example calculation. When a cannon is fired, the cannon ball gains forward momentum and the cannon gains backward momentum. In some cases momentum is conserved but energy is definitely not. Conservation of momentum is derived from […] The first part is clear and unequivocal. Conservation of momentum explains why a gun or cannon recoils backwards when it is fired. This makes momentum conservation a fundamental tool for analyzing collisions. If the external forces are zero then the momentum doesn't change - that's the definition of force. All of Work, Energy, and Energy Resources is devoted to momentum, and momentum has been important for many other topics as well, particularly where collisions were involved. Momentum should not be confused with energy. External forces acting on the system . Since momentum is a vector, we need to check the net force in each direction, and if the net force is zero then the momentum for that direction is conserved. use cons of energy for questions that involve gravity, springs, and kinetic energy (where energy is moved between those types of energy). Conservation of momentum holds for a system when there are no external forces on the system. the principle that the total linear momentum in a closed system is constant and is not affected by processes occurring inside the system We will see that momentum has the same importance in modern physics. Use the conservation of momentum to equate the momentum after the collision to the momentum before the collision. This is because the momentum lost by one object is equal to the momentum gained by the other. Full details of the calculation are shown in the video at the top of this article.